Thorax of hexapods

Definition

The middle tagma (body region) of hexapods, located between the head and abdomen, bearing the three pairs of legs and, in winged insects, one or two pairs of wings. In hexapods, the thorax comprises three segments—prothorax, mesothorax, and metathorax—each with a dorsal tergum, lateral pleuron, and ventral sternum. The thorax is primarily locomotory in function and, unlike the abdomen, lacks reproductive or digestive appendages (excepting certain basal hexapods). In winged insects, the mesothorax and metathorax are often enlarged and modified to accommodate flight musculature.

Etymology

From Greek thōrax, breastplate or chest

Example

In a grasshopper (Orthoptera), the greatly enlarged mesothorax and metathorax house the powerful indirect flight muscles that deform the thoracic box to power the wings; the prothorax remains smaller and bears only the first pair of walking legs.

Synonyms

• pterothorax (when referring to wing-bearing segments specifically)

Related Terms

• prothorax
• Mesothorax
• Metathorax
• tagma
• pleuron
• sternum
• Tergum
• Coxa
• indirect flight muscle
• petiole
• propodeum

Usage Notes

The term 'thorax' in hexapods is distinct from the broader usage in other arthropods: in arachnids, the cephalothorax (prosoma) combines head and thoracic elements, while crustacean thoraxes vary widely in segment number. Specialists distinguish the 'pterothorax' (meso- plus metathorax) when discussing winged insects specifically. In some groups—particularly Hymenoptera—the first abdominal segment (propodeum) is fused to the thorax, creating a 'mesosoma' that can confuse segmental counts. The thorax is never the site of spiracles in adult insects (unlike the abdomen), a useful diagnostic distinction.